This invention relates to a solid state circuit device for controlling the amount of voltage, current or power to heating, electromotive, and other devices. The device may be used for motor speed controls, in furnaces, for petrochemical plants, in heaters, and in glass processing plants, etc. The device also has particular application for turn-on switching at precise, finely divided time intervals.
Historically, the control of A.C. power has evolved from simple on-off switching to transformer tap changing, and then to thyratrons and SCRs. Initially, the control response time base was in the order of minutes. The subsequent use of discrete and integrated solid state devices has reduced this time base to fractions of a second. However, many types of processes are quite complex, and require a more precise degree of control. Moreover, present day devices are sensitive to noise, frequency, and temperature changes. Hence, digital control techniques are preferred since they are much less sensitive to temperature variations and induced noise.
In addition, it would be desireable to employ an inherently stable digital device that would subdivide a power control cycle into a discrete number of periods. The device could then be turned on within a very closely controlled time period based on a digital selection code. This digital code could be obtained directly from a computer or converted from an analogue command signal. Thus, if a half cycle of power were to be subdivided by a factor of 2.sup.n, e.g., 256, it would enable the device to be turned on at any one of the precisely defined 256 intervals. Of course, the power cycle could be divided into fewer or greater numbered intervals of 2.sup.n, depending on the required control resolution.
It would be desireable to provide an operator closed loop, an automatic closed loop, or a computer automated control for the device. This would enable the device to control the process manually, automatically, or by a computer using voltage, current, or power feedback.
It would also be desireable to provide a means for tailoring the response of the device to its control command signal to obtain a desired operating characteristic. One example would be to vary or change the output response so that the command signal could directly require a percentage of RMS current, or in another case, the percentage of average power.